The present invention relates to a process for the preparation of a collidine and 2,3,5,6-tetramethyl pyridine over a zeolite catalyst. More particularly, the present invention relates to a process for the preparation of collidine from methyl ethyl ketone, formaldehyde and ammonia in gas phase in an eco-friendly method with high yields and selectivity.
The present invention provides a non-corrosive, eco-friendly process where the catalyst can be recycled and reused for many times. The process involves no-wastage of the compounds (i.e. high atom selectivity) and has high selectivity of products.
Collidines and tetramethyl pyridines are useful compounds that find application as raw materials in various medicines like Omeprazole and also in agrochemicals.
The following processes are known for the preparation of 2,3,5-collidine by reacting a carbonyl compound with ammonia in a gas phase in the presence of a catalyst.
A process comprising reacting methacrolein and methyl ethyl ketone with ammonia in a gas phase in presence of catalyst which comprises silica-alumina containing at least one element selected from the group consisting of cobalt, zinc, cadmium and lead. (U.S. Pat. No. 6,111,113) wherein the yield of 2,3,5-collidine is 42% and other major product is 2-methyl 5-ethyl pyridine.
A process comprising reacting methacrolein with ammonia in a gas phase in the presence of a catalyst which comprises silicon and an element such as zirconium, aluminium and/or phosphorus (see JP-A-8-245589), and a process comprising reacting methacrolein and methyl ethyl ketone with ammonia in a gas phase in the presence of an oxide catalyst comprising silicon, phosphorus and/or boron (see JP-A-8-259537). However, the main product of the former process is 3,5-lutidine, while 2,3,5-collidine is obtained in a yield of only 16.5%. In the latter process, 2,3,5-collidine is obtained in a yield of 15 to 37%.
Another process by Yamaji; Mitsuharu comprises the synthesis of collidine along with tetramethyl pyridine by the alkylation of 3,5-lutidine with aliphatic alcohols over Raney nickel catalysts. The yields and selectivities are higher but the process suffers from use of hydrogen and is not a continuous process.
Some processes have also been reported such as the separation of 2,3,5-collidine from tar and the reacting the 3,5-lutidine methyl lithium butyl but these or not upto the level of commercial use. In the latter process the yield is upto 80% but suffers in industrial operation.
Syntheses of pyridine bases have been extensively studied over the solid acid catalysts and on some zeolites, but those are not for the synthesis of collidines and tetramethyl pyridines. Prior art describes neither a gas phase catalytic reaction of a carbonyl compound with formaldehyde and ammonia for the preparation of collidines, nor the use of crystalline uniform porous zeolite catalysts.
The main object of the present invention is to provide a process for the synthesis of collidines over zeolite molecular sieves in a heterogeneous eco-friendly method.
Another object of the present invention is to provide a process for the preparation of collidines, in a high yield and at the same time, 2,3,5,6-tetramethyl pyridine, by the gas phase catalytic reaction of a carbonyl compound and ammonia.
Another object of the present invention is to provide a process for synthesizing collidines using a specific zeolite catalyst.
Another object of the invention is to provide a process wherein the catalyst is capable of reuse and recycling.
Accordingly the present invention relates to a process for the preparation of a compound of formula 1 
wherein R1 and R3 are H or methyl, R2 and R4 are methyl, said process comprising reacting methyl ethyl ketone with formaldhyde in the presence of ammonia in gas phase conditions in the presence of a catalyst comprising zeolite ZSM-5.
In one embodiment of the invention, R1 is methyl and R3 is H to give 2,3,5-collidine.
In one embodiment of the invention R1 is H and R3 is methyl to give 2,3,6-collidine.
In one embodiment of the invention R1 and R3 are methyl to give 2,3,5,6-tetramethylcollidine.
In one embodiment of the invention, the ZSM-5 catalyst contains at least one element selected from the group consisting of lanthanum, lead, manganese, iron, copper and cobalt.
In one embodiment of the invention A process as claimed in claim 1, wherein the Si/Al ratio in the catalyst is in the range of 15 to 140.
In one embodiment of the invention the amount of said at least one element is between 1 and 10 wt. % of metal to said zeolite.
In one embodiment of the invention said at least one element is contained in the form of an oxide in said zeolite ZSM-5.
In one embodiment of the invention the amount of methyl ethyl ketone is at least one mole per one mole of formaldehyde, and the amount of ammonia is between 0.5 and 5 moles per one mole of the total of methyl ethyl ketone and formaldehyde.
In one embodiment of the invention, wherein methanol is added to the starting materials in an amount of up to 0.5 mole per mole of ethyl methyl ketone.
The process of the present invention uses ZSM-5 silica-alumina containing at least one element selected from the group consisting lanthanum, lead; manganese, iron and copper, as a catalyst. At least one element selected from the group consisting of lanthanum, lead, manganese, iron and copper is contained in the catalyst in the form of a metal, an ion and/or a compound. Examples of the compounds of such elements are inorganic compounds such as their oxides, nitrates, hydroxides, carbonates, etc.
The content of alumina in the zeolite varies between 15 and 140 of the silica/alumina ratio. The amount of lanthanum, manganese, iron and copper and/or lead is usually between 1 and 10 wt. %, preferably between 2 and 5 wt. %, based on the weight of the ZSM-5 catalyst. When the content of the alumina and the amount of the element or elements are within the above ranges, respectively, 2,3,5-collidine and 2,3,6-collidine are obtained in good yields, and the deterioration of the catalytic activity of the catalyst with time is suppressed. The presence of water in the formaldehyde decreases the coke formation and increases the lifetime of the catalyst. The catalyst is be prepared from at least one compound selected from the group consisting of inorganic compounds of the above elements (e.g. oxides, halides, sulfates, nitrates, hydroxides, sulfides, silicates, titanates, carbonates, etc.) or organometal compounds of the above elements (e.g. carboxylate salts, organic chelates, etc.) as the source materials for the elements, ZSM-5 catalyst was used as commercial catalyst and it can be prepared by silicon compound (e.g. water glass, silica sol, alkali silicates, etc.) and an aluminium compound (e.g. aluminium nitrate, aluminium sulfate, alumina sol, etc.) in presence of template tripropyl amine by reported literature.
The catalyst of the present invention may be prepared by any conventional method such as impregnation, ion exchange, and the like. For example, ZSM-5 impregnated with the aqueous solution of the above compound(s) of the element(s), dried and calcined. The ZSM-5(30) powder is ion exchanged by treating the powder with the aqueous solution of metal salt solutions of the compound(s) of the element(s) lanthanum, cobalt, lead, copper, manganese and iron and after that it is washed with water, dried and calcined. In the process of the present invention, the catalyst can be used as a fixed bed catalyst or a fluidized bed catalyst.
When the catalyst of the present invention is used as the fixed bed catalyst, it is shaped in the form of a solid tablet with a tabletting machine and then meshed to the 18-30 size. In either case, the shaped catalyst is calcined at a temperature of between 400 and 600xc2x0 C. for several hours in the atmosphere of air to strengthen the catalyst and to evaporate volatile components off. The calcination of the catalyst after shaping is not always necessary, since the catalyst is heated in a reactor for the gas-phase catalytic reaction.
The process of the present invention can be carried out by any conventional method. For example, a gaseous mixture of methyl ethyl ketone, formaldehyde and ammonia is supplied over the catalyst of the present invention, and catalytically reacted in the gas phase. The amount of methyl ethyl ketone is usually at least one mole, preferably between 0.5 and 2 moles per one mole of formaldehyde. The amount of ammonia is usually between 0.5 and 5 moles, preferably between 1 and 3 moles, per one mole of the total of formaldehyde and methyl ethyl ketone. The reaction temperature for the gas phase catalyst reaction is usually in the range between 300 and 450xc2x0 C., preferably in the range between 350 and 400xc2x0 C. The weight hourly space velocity of the mixture of the raw materials is usually between 0.25 and 1 hxe2x88x921. The reaction pressure may be reduced pressure, atmospheric pressure or elevated pressure. Preferably, the reaction pressure is atmospheric pressure.
The possible reaction scheme of the process of the invention is given below: 
When the process of the present invention is performed in the form of a fixed bed, the catalyst of the present invention is packed in a reactor and heated to a reaction temperature. Then, the mixture of methyl ethyl ketone, formaldehyde and ammonia was allowed to proceed through the gas phase catalytic reaction while maintaining the suitable reaction temperature. Thus, a reaction product containing 2,3,5-collidine, 2,3,6-collidine and 2,3,5,6-tetramethylpyridine is obtained.
The reaction product gas is condensed by cooling, or trapped in a suitable solvent. The condensed or trapped liquid contains 2,3,5-collidine, 2,3,6-collidine and 2,3,5,6-tetramethyl pyridine along with the lutidines and other pyridine derivatives.